1. Field of the Invention
This invention relates to a method of making a copper wire rod with low semi-softening temperature by continuous casting and rolling method, a method of making a copper wire by using the copper wire rod made by the above method, and a copper wire with low semi-softening temperature made by the above method.
2. Description of the Related Art
Most of copper wires including electrical copper wires are manufactured by a method using the continuous casting and rolling method. The manufacturing method is conducted such that a relatively large copper wire rod is made using a copper melt obtained by melting a raw material copper, the copper wire rod as a matrix material is drawn in separate processing and appropriately annealed to have a copper wire with a predetermined size.
In the continuous casting and rolling method, the copper melt is obtained by melting the raw material copper such as electrolytic copper, scrap copper etc. in a melting furnace such as a shaft kiln, and supplied to a continuous casting machine of belt caster type via a transfer gutter and a retaining furnace etc. for continuous casting. Then a cast bar thus obtained is hot-rolled and then cooled to have a copper wire rod with a predetermined size. Thus, the continuous casting and rolling method can be implemented with continuous lines for melting, casting and rolling the raw material copper, so that it is very efficient and excellent in productivity as the production method of copper wire rod.
The copper wire rods to be obtained by the continuous casting and rolling include a oxygen-free copper wire and a tough pitch copper wire. However, the oxygen-free copper wire is generally difficult to make industrially by using the continuous casting and rolling method. For example, technical problems reside in the selection of the raw material copper (i.e., scrap copper cannot be used because of its high oxygen content), and in that all steps have to be strictly kept at non-oxidation atmosphere (otherwise, oxygen will be absorbed in the melt to increase the oxygen concentration so as not to gain the oxygen-free copper). Thus, producing the oxygen-free copper by the continuous casting and rolling method is apparently disadvantageous in cost as compared to producing the tough pitch copper by the continuous casting and rolling method.
On the other hand, when the copper wire is produced such that the copper wire rod made by the continuous casting and rolling method is used as a matrix material, drawn and annealed, the productivity of the copper wire can be enhanced by conducting continuously the drawing and annealing steps. In this case, it is very important to use a wire drawing matrix material (i.e., copper wire rod) with low softening temperature.
In other words, where the drawing step and the annealing step are continuously conducted, if the annealing temperature of the wire drawing matrix material increases, problems arise that it takes a long time for the annealing and that the production speed at the drawing step has to be synchronous with the prolonged production speed at the annealing step so that the productivity of the copper wire lowers. In addition, if the annealing temperature of the wire drawing matrix material increases, thermal energy needed for the annealing increases such that an increase in the product cost is caused. Thus, in order to inexpensively produce the copper wire by the excellent productivity method, it is very important to use a copper wire rod with low annealing temperature, i.e., low softening temperature as the wire drawing matrix material.
In order to decrease the softening temperature of a copper material including the copper wire rod, impurity elements included in the copper material has to be removed to increase the copper purity of the copper material. For example, methods for removing the impurity elements included in the copper material may include selecting the raw material copper (i.e., using high-purity copper), oxidation refining (or reduction refining) of copper melt obtained by melting the raw material copper etc.
However, these methods for removing the impurity elements all have a problem that a considerable increase in the product cost is caused. Although the oxidation refining or reduction refining are one of options that can be considered when tough pitch copper is used as the raw material copper, they are economically disadvantageous in terms of the casting technique and unsuitable industrially. Meanwhile, the production method of copper wire rod by the continuous casting and rolling method is very advantageous industrially and economically since good scrap copper containing tough pitch copper can be used (or recycled) as the raw material copper.
It is known that the alternative method for reducing the softening temperature of a copper material is to lower the concentration of some kind of element of the impurity elements included in the copper material. The some kind of element may be sulfur (S) that can be included as being solved in copper. A method of vacuum degassing the copper melt is tried for reducing the concentration of sulfur solved in copper. However, in this method, it is impossible to sufficiently decrease (or deposit) the sulfur solved in copper so that the softening temperature of the copper material cannot be reduced sufficiently.
JP-A-2006-272422 and JP-A-2006-274383 disclose a method of making a copper material (copper wire rod) with low softening temperature, wherein a metal (sulfur affinity metal) with a strong affinity for sulfur containing a metal or an alloy selected from Nb, Ti, Zr, V, Ta, Fe, Ca, Mg and Ni is added at a predetermined rate to a copper melt using the tough pith copper as a raw material copper, and the added metal is reacted with sulfur (S) included in the melt of tough pitch copper for depositing the sulfur as a sulfide such that the concentration of sulfur solved in the copper is reduced to lower the softening temperature of the copper material.
JP-A-2007-046102 discloses a method of making an oxygen-free copper wire with low annealing softening start temperature and semi-softening temperature, wherein an oxygen-free copper melt using a specific oxygen-free copper as a raw material copper is continuously cast into an ingot with a large cross section, the ingot is then hot-rolled into a wire rod, the wire rod is subjected to reduction in area to form a lot of sites with high strain as recrystallization nuclei.
However, as mentioned earlier, in the method of vacuum degassing the copper melt for reducing the concentration of sulfur solved in copper, it is impossible to sufficiently decrease (or deposit) the sulfur solved in copper so that the softening temperature of the copper material cannot be reduced sufficiently.
Although JP-A-2006-272422 and JP-A-2006-274383 disclose the method that a metal (sulfur affinity metal) with a strong affinity for sulfur containing a metal such as Nb is added at a predetermined rate to a copper melt using the tough pith copper, and the added metal is reacted with sulfur (S) included in the copper material for depositing the sulfur as a sulfide such that the concentration of sulfur solved in the copper is reduced to lower the softening temperature of the copper material, it is necessary to add the metal with the strong affinity for sulfur to the copper melt.
Although JP-A-2007-046102 discloses the method that the oxygen-free copper melt is continuously cast into the ingot with a large cross section, it is necessary to use the expensive oxygen-free copper as a raw material.